To obtain hydrocarbons such as oil and gas from subterranean formations, wellbores are drilled into the formations by rotating a drill bit attached to an end of a drill string. A substantial portion of current drilling activity involves what is referred to in the art as “directional” drilling. Directional drilling involves drilling deviated and/or horizontal wellbores (as opposed to straight, vertical wellbores). Modern directional drilling systems generally employ a bottom hole assembly (BHA) at the end of the drill string that includes a drill bit and a hydraulically actuated motor to drive rotation of the drill bit. The drill bit is coupled to a drive shaft of the motor, typically through an assembly configured for steering the path of the drill bit, and drilling fluid pumped through the motor (and to the drill bit) from the surface drives rotation of the drive shaft to which the drill bit is attached. Such hydraulic motors are commonly referred to in the drilling industry as “mud motors,” “drilling motors,” and “Moineau motors.” Such motors are referred to hereinafter as “hydraulic drilling motors.”
Hydraulic drilling motors include a power section that includes a stator and a rotor disposed in the stator. The stator may include a metal housing having an interior lined with a helically contoured or lobed elastomeric material, which material is formulated to wear. The elastomeric material may be replaced after a certain amount of use, or when a selected amount of wear or damage is detected. The rotor is usually made from a suitable metal, such as steel, and configured with an outer lobed surface. Pressurized drilling fluid (commonly referred to as “drilling mud”) is pumped into a progressive cavity formed between the rotor and the stator lobes. The force of the pressurized fluid pumped into and through the cavity causes the rotor to turn in a planetary-type motion. An output shaft connected to the rotor via a flexible coupling compensates for eccentric movement of the rotor. The output shaft is coupled to a bearing assembly supporting a drive shaft (also referred to as a “drive sub”), which in turn rotates the drill bit through the aforementioned steering assembly.
As drilling fluid flows through the progressive cavity between the rotor and the stator, forces on the rotor and the stator, as well as abrasives and chemicals in the drilling fluid, can damage parts of the motor.